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Log/antilog amplifiers to multiply two voltages from single supply.

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s3c:
I'm trying to multiply two voltages together using log/antilog amplifiers. The idea is to log the two voltages, add them together and then antilog them, I have it working with ideal op amps but can't seem to get the single supply version working. Here's a screenshot of the ideal:



I was hoping I could just replace all the grounds with a virtual ground for the single supply version but this doesn't work, the antilog stage fails if I do this and I don't really know why:



I've attached the ltspice files in a zip, I'd appreciate if someone with a little more experience and insight could take a look.

scrat:
At a first look, it seems the diode is off, since adding a positive voltage you're polarizing it backwards.

If it's for a real application, some time ago Dave posted about a limitation opamp (or a similar name) from AD, which could be easily used to make a "linear" multiplier. Is this an option?

s3c:
I was hoping to use it as a power monitor where I got a proportional output signal by multiplying two voltages representing current and voltage. Alternative sollutions are fine but on the other hand I'd like to know why it doesn't work as well. I've measured the voltages over the diodes for both cases and they're fairly identical.

Floyo:
If you don't want to build the whole thing yourself you could have a look at the AD633, it's an analog multiplier with fully differential inputs so measuring from different grounds isn't a problem. I've used it in a prototype spectrum analyser for a school project, and it works just fine, much better then RF multipliers (NE612). The problem with diy log amps is that they need to be temperature compensated and the diodes need to be properly matched otherwise they become inaccurate, there's a paragraph on that in the art of electronics somewhere.

scrat:

--- Quote from: s3c on July 14, 2011, 01:42:22 pm ---I've measured the voltages over the diodes for both cases and they're fairly identical.

--- End quote ---

Do you mean the antilog diode is forward biased? It doesn't seem so, since current should flow on the reverse direction, to have a positive output on the opamp.

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